1. Field of the Invention
The present invention relates to an image processing device, an imaging device, an image processing method, and non-transitory computer readable recording medium storing a program regarding restoration processing based on a point spread function.
2. Description of the Related Art
In an object image captured through an optical system, a so-called point spread phenomenon in which a point object has minute spread due to the influence of diffraction, aberration, or the like caused by the optical system may be observed. A function representing a response to a point light source of an optical system is called a point spread function (PSF), and is known as a parameter responsible for resolution deterioration (blurring) of a captured image.
The captured image deteriorated in image quality due to the point spread phenomenon can be recovered in image quality through point image restoration processing based on the PSF. The point image restoration processing is processing in which a deterioration characteristic (point image characteristic) caused by aberration or the like of a lens (optical system) is determined in advance, and point spread of the captured image is cancelled or reduced through image processing using a restoration filter (recovery filter) according to the point image characteristic.
Various methods for the point image restoration processing have been suggested, and for example, JP2011-059813A discloses an image processing device which performs image recovery for an image subjected to nonlinear correction using blind deconvolution. This image processing device comprises a correction unit which performs correction for reducing nonlinear gradation correction for a captured image subjected to nonlinear gradation correction, and an image recovery unit which performs image recovery by applying blind deconvolution to the captured image with reduced gradation correction.
JP2013-020610A discloses an image processing device which reduces over-recovery of image data by image recovery processing. In this image processing device, image recovery processing is performed for color image data in an RGB format before gamma processing, the difference between amplification and attenuation of a pixel signal value by gamma correction is absorbed, and a limit value of a variation is calculated such that the maximum value of the variation of the pixel signal value becomes constant even after gamma correction. With this, the technical problems in that “a situation in which a deterioration state of image data actually obtained does not match a deterioration state of image data to be recovered by an image recovery filter occurs due to a saturated pixel”, and “image quality deterioration, such as undershoot or overshoot, occurs in an edge portion, and in particular, undershoot in a low brightness portion is amplified by gamma processing after image recovery processing” have been solved.
The point spread function of the optical system is used in a restoration technique of an image with an extended depth of focus, and JP2012-049759A discloses an imaging module which executes image restoration in a short period of time with excellent accuracy. In this imaging module, restoration processing is applied to a brightness signal after synchronization processing (demosaic processing), whereby it is not necessary to separately provide parameters of the restoration processing for RGB, and it is possible to accelerate the restoration processing. Furthermore, adjacent pixels are put together in a predetermined unit and the common restoration processing parameter is applied to this unit to perform a deconvolution processing, whereby the accuracy of the restoration processing is improved.
In addition to the point image restoration processing, image processing for improving image quality may be performed, and for example, the sharpness of an image can be improved by sharpening processing (contour enhancement processing, edge enhancement processing, sharpness enhancement processing).
As a specific processing method relating to the point image restoration processing and the sharpening processing described above, for example, JP2006-333061A discloses that “camera shake correction for performing edge enhancement processing after image restoration processing using an image restoration filter is performed” and “camera shake correction for merely performing edge enhancement processing” is performed. Furthermore, JP2006-129236A discloses that filters having different restoration intensities are selected according to edge intensity, thereby changing the intensity of the restoration filter itself according to edge intensity.